1. Field of the Invention
The present invention relates to an taking lens system, and more particularly to a high-performance, compact taking lens system suitable for use in a digital input device (such as a digital still camera or digital video camera) that captures an image of a subject with a solid-state image sensor.
2. Description of Related Art
In recent years, as personal computers and the like become more and more popular, digital still cameras and digital video cameras (hereinafter collectively referred to as digital cameras), which permit easy entry of image data into digital devices, have been becoming more and more popular for personal use among general users. Such digital cameras are expected to become even more widespread as image data input devices in the future.
Solid-state image sensors, such as CCDs (charge-coupled devices), used in digital cameras have been made increasingly small, and accordingly further miniaturization is sought in digital cameras themselves. As a result, miniaturization is eagerly sought also in taking lens systems, which occupy the largest volumes in digital input devices. The easiest way to make an taking lens system smaller is to make a solid-state image sensor smaller. However, this involves making photosensitive elements smaller, and thus makes the solid-state image sensor more difficult to fabricate, and in addition requires higher performance in the taking lens system.
On the other hand, making an taking lens system smaller while keeping the size of a solid-state image sensor unchanged inevitably brings the exit pupil position closer to the image plane. When the exit pupil position is brought closer to the image plane, the off-axial rays exiting from the taking lens system are obliquely incident on the image plane. This makes it impossible to make the most of the light-condensing ability of the micro lenses provided in front of the solid-state image sensor, with the result that the obtained image shows extremely uneven brightness between in a central portion and in a peripheral portion thereof. This can be avoided by bringing the exit pupil position of the taking lens system farther away from the image plane, but this inevitably makes the taking lens system as a whole unduly large.
Furthermore, in recent years, in keen competition for lower prices, lower costs have been increasingly eagerly sought in taking lens systems. Moreover, in recent years, as increasingly high densities are sought in solid-state image sensors, increasingly high performance has been sought in taking lens systems. To meet such requirements, Patent Publications 1 to 4 listed below, among others, propose taking lens systems for use with a solid-state image sensor that have an aperture stop disposed in front thereof and that are composed of three, namely a positive, a positive, and a negative, lens elements.
Patent Publication 1: US-2004-179275-A1
Patent Publication 2: JP-A-2004-226487
Patent Publication 3: JP-A-2004-252312
Patent Publication 4: JP-A-2004-309695
Disadvantageously, however, the three-lens-element taking lens systems proposed in Patent Publications 1 to 4 mentioned above suffer from an improper shape of the first lens element and an improper relationship between the optical powers of the first and second lens elements. As a result, these constructions are extremely sensitive to manufacturing errors relative to the lens optical axis, and therefore tend to be difficult to manufacture with satisfactory peripheral performance.